SIPPING Working Group T. Sawada
Internet Draft KDDI Corporation
Intended status: Informational P. Kyzivat
Expires: September 2008 Cisco Systems, Inc.
March 31, 2008
SIP (Session Initiation Protocol) Usage of the Offer/Answer Modeldraft-ietf-sipping-sip-offeranswer-07.txt
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Abstract
The Session Initiation Protocol (SIP) utilizes the offer/answer
model to establish and update multimedia sessions using the Session
Description Protocol (SDP). The description of the offer/answer
model in SIP is dispersed across multiple RFCs. This document
summarizes all the current usages of the offer/answer model in SIP
communication.
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Acknowledgment................................................... 261. Introduction
SIP utilizes the offer/answer model to establish and update the
session. The rules to govern the offer/answer behaviors in SIP are
described in the several RFCs.
The primary purpose of this document is to describe all forms of
SIP usage of the offer/answer model in one document to help the
readers to fully understand it. Also, this document tries to
incorporate the results of the discussions on the controversial
issues to avoid repeating the same discussions later.
This document is not intended to make normative changes. Rather, it
makes the remaining open issues clear and leaves them for further
study.
2. Summary of SIP usage of the Offer/Answer Model
The offer/answer model itself is independent from the higher layer
application protocols which utilize it. SIP is one of the
applications using the offer/answer model. RFC 3264 [3] defines the
offer/answer model, but does not specify which SIP messages should
convey an offer or an answer. This should be defined in the SIP
core and extensions RFCs.
In theory, any SIP message can include a session description in its
body. But a session description in a SIP message is not necessarily
an offer or an answer. Only certain session description usages that
conform to the rules described in standards-track RFCs can be
interpreted as an offer or an answer. The rules for how to handle
the offer/answer model are currently defined in several RFCs.
The offer/answer model defines a mechanism for update of sessions.
In SIP, a dialog is used to associate an offer/answer exchange with
the session which it is to update. In other words, only the
offer/answer exchange in the SIP dialog can update the session
which is managed by that dialog.
2.1. Offer/Answer Exchange Pairs in SIP Messages
Currently, the rules on the offer/answer model are defined in RFC3261 [1], RFC 3262 [2] and RFC 3311 [4]. In these RFCs, only the
six patterns shown in Table 1 are defined for exchanging an offer
and an answer with SIP messages.
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Note that an offer/answer exchange initiated by an INVITE request
must follow exactly one of the patterns 1, 2, 3, 4. When an initial
INVITE causes multiple dialogs due to forking, an offer/answer
exchange is carried out independently in each distinct dialog. When
an INVITE request contains no offer, only pattern 2 or pattern 4
apply. 'The first reliable non-failure message' must have an offer
if there is no offer in the INVITE request. This means that UA
which receives the INVITE request without an offer must include an
offer in the first reliable response with 100rel extension. If no
reliable provisional response has been sent, the UAS must include
an offer when sending 2xx response.
In pattern 3, the first reliable provisional response may or may
not have an answer. When a reliable provisional response contains a
session description, and is the first to do so, then that session
description is the answer to the offer in the INVITE request. The
answer can not be updated, and a new offer can not be sent in a
subsequent reliable response for the same INVITE transaction.
In pattern 5, a PRACK request can contain an offer only if the
reliable response which it acknowledges contains an answer to the
previous offer/answer exchange.
NOTE: It is legal to have UPDATE/2xx exchanges without
offer/answer exchanges (pattern 6). However when re-INVITEs
are sent for non-offer/answer purposes, an offer/answer
exchange is required. In that case the prior SDP will
typically be repeated.
There may be ONLY ONE offer/answer negotiation in progress for a
single dialog at any point in time. Section 4 explains how to
ensure this. When an INVITE results in multiple dialogs each has a
separate offer/answer negotiation.
NOTE: This is when using a Content-Disposition of "session".
There may be a second offer/answer negotiation in progress
using a Content-Disposition of "early-session" [6]. That is
not addressed by this draft.
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Offer Answer RFC Ini Est Early
-------------------------------------------------------------------
1. INVITE Req. 2xx INVITE Resp. RFC 3261 Y Y N
2. 2xx INVITE Resp. ACK Req. RFC 3261 Y Y N
3. INVITE Req. 1xx-rel INVITE Resp. RFC 3262 Y Y N
4. 1xx-rel INVITE Resp. PRACK Req. RFC 3262 Y Y N
5. PRACK Req. 200 PRACK Resp. RFC 3262 N Y Y
6. UPDATE Req. 2xx UPDATE Resp. RFC 3311 N Y Y
Table 1. Summary of SIP Usage of the Offer/Answer Model
In Table 1, '1xx-rel' corresponds to the reliable provisional
response which contains the 100rel option defined in RFC 3262 [2].
The 'Ini' column shows the ability to exchange the offer/answer to
initiate the session. 'Y' indicates that the pattern can be used in
the initial offer/answer exchange, while 'N' indicates that it can
not. Only the initial INVITE transaction can be used to exchange
the offer/answer to establish a multimedia session.
The 'Est' column shows the ability to update the established
session.
The 'Early' column indicates which patterns may be used to modify
the established session in an early dialog. There are two ways to
exchange a subsequent offer/answer in an early dialog.
2.2. Rejection of an Offer
It is not entirely clear how to reject an offer when it is
unacceptable, and some methods do not allow explicit rejection of
an offer. For each of the patterns in Table 1, Table 2 shows how to
reject an offer.
When a UA receives an INVITE request with an unacceptable offer, it
should respond with a 488 response, preferably with Warning header
field indicating the reason of the rejection, unless another
response code is more appropriate to reject it. (Pattern 1 and
Pattern 3)
When a UA receives an UPDATE request with an offer which it can not
accept, it should respond with a 488 response preferably with
Warning header field indicating the reason of the rejection, unless
another response code is more appropriate to reject it. (Pattern 6)
When a UA receives a PRACK request with an offer which it can not
accept, it may respond with a 200 response with a syntactically
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correct session description. This may optionally be followed by an
UPDATE request to rearrange the session parameters if both ends
support the UPDATE method. Alternatively the UA may terminate the
dialog and send an error response to the INVITE request. The
validity and consequences of a 488 response to PRACK is an open
issue which is discussed within a sequent section. (Pattern 5)
When a UA receives a response with an offer which it can not accept,
the UA does not have a way to reject it explicitly. Therefore, a UA
should respond to the offer with the correct session description
and rearrange the session parameters by initiating a new
offer/answer exchange, or alternatively terminate the session.
(Pattern 2 and Pattern 4) When initiating a new offer/answer, a UA
should take care not to cause an infinite offer/answer loop.
Offer Rejection
-----------------------------------------------------
1. INVITE Req. 488 INVITE Response
2. 2xx INVITE Resp. Answer in ACK Req. followed by new offer
OR termination of dialog
3. INVITE Req. 488 INVITE Response (same as Pattern 1.)
4. 1xx-rel INVITE Resp. Answer in PRACK Req. followed by new offer
5. PRACK Req. (*) 200 PRACK Resp. followed by new offer
OR termination of dialog
6. UPDATE Req. 488 UPDATE Response
Table 2. Rejection of an Offer
(*) A UA should only use PRACK to send an offer when it has strong
reasons to expect the receiver will accept the offer.
2.3. Session Description which is not Offer nor Answer
As previously stated, a session description in a SIP message is not
necessarily an offer or an answer. For example, SIP can use a
session description to describe capabilities apart from
offer/answer exchange. Examples of this are 200 OK responses for
OPTIONS and 488 responses for INVITE.
3. Detailed Discussion of the Offer/Answer Model for SIP3.1. Offer/Answer for the INVITE method with 100rel extension
The INVITE method provides the basic procedure for offer/answer
exchange in SIP. Without the 100rel option, the rules are simple as
described in RFC 3261 [1]. If an INVITE request includes a session
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description, pattern 1 is applied and if an INVITE request does not
include a session description, pattern 2 is applied.
With 100rel, pattern 3 and pattern 4 are added and this complicates
the rules. An INVITE request may cause multiple responses. Note
that even if both UAs support the 100rel extension, not all the
provisional responses may be sent reliably. Note also that a
reliable provisional response is allowed without a session
description if the UAS does not wish to send the answer yet. An
unreliable provisional response may include a session description
in the body if the UAS has not sent a reliable response, but its
session description is neither an offer nor an answer. All the
session descriptions in the unreliable responses to the INVITE
request must be identical to the answer which is included in the
reliable response. A session description in an unreliable response
that precedes a reliable response can be considered a "preview" of
the answer that will be coming, and hence may be treated like an
answer until the actual one arrives.
NOTE: This "preview" session description rule applies to a
single offer/answer exchange. In parallel offer/answer
exchanges (caused by forking) a UA may obviously receive a
different "preview" of an answer in each dialog. UAs are
expected to deal with this.
Although RFC 3261 says a UA should accept media once an INVITE with
an offer has been sent, in many cases, an answer (or, at least a
preview of it) is required in order for media to be accepted.
Therefore, a UAS should send an SDP answer reliably (if possible)
before it starts sending media. And, if neither the UAC nor the UAS
support 100rel, the UAS should send a preview of the answer before
it starts sending media.
3.1.1. INVITE Request with SDP
When a UAC includes an SDP body in the INVITE request as an offer,
it expects the answer to be received with one of the reliable
responses. Other than that, no offer/answer exchanges can occur in
the messages within the INVITE transaction.
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UAC UAS
| F1 INVITE (SDP) | <- The offer in the offer/answer model
|-------------------->|
| F2 1xx (SDP) | <- The offer/answer exchange is not
|<--------------------| closed yet, but UAC acts as if it
| | ^ receives the answer.
| F3 1xx-rel (no SDP) | |<- a 1xx-rel may be sent without answer
|<--------------------| | SDP.
| F4 PRACK (no SDP) | |
|-------------------->| | UAC must not send a new offer.
| F5 2xx PRA (no SDP) | |
|<--------------------| v
| |
| F6 1xx-rel (SDP) | <- The answer in the offer/ answer model
|<--------------------| -
| F7 PRACK | | UAC can send a new offer in a PRACK
|-------------------->| | request to acknowledge F6.
| F8 2xx PRA | | After F7 UAC and UAS can send a new
|<--------------------| v offer in an UPDATE request.
| |
| F9 1xx-rel | <- SDP should not be included in the
|<--------------------| subsequent 1xx-rel once offer/answer
| F10 PRACK | has been completed.
|-------------------->|
| F11 2xx PRA |
|<--------------------|
| |
| F12 2xx INV | <- SDP should not be included in the
|<--------------------| final response once offer/answer has
| F13 ACK | been completed.
|-------------------->|
Figure 1 Example of Offer/Answer with 100rel Extension (1)
For example, in Figure 1, only the SDP in F6 is the answer. The SDP
in the non-reliable response (F2) is the preview of the answer and
must be the same as the answer in F6. Receiving F2, the UAC should
act as if it receives the answer. However, offer/answer exchange is
not completed yet and the UAC must not send a new offer until it
receives the same SDP in the first reliable response, which is the
real answer. After sending the SDP in F6, the UAS must prepare to
receive a new offer from the UAC with an UPDATE request or a PRACK
request.
The UAS does not include SDP in responses F9 and F12. However, the
UAC should prepare to receive SDP bodies in F9 and/or F12, and just
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ignore them, to handle a peer that does not conform to the
recommended implementation.
3.1.2. INVITE request without SDP
When a UAC does not include an SDP body in the INVITE request, it
expects the offer to be received with the first reliable response.
The UAC will send the answer in the request to acknowledge the
response, i.e. PRACK or ACK request of the reliable response. Other
than that, no offer/answer exchanges can occur in the messages
within the INVITE transaction.
NOTE: The UAS should not include SDP in the responses F6 and
F9. However, the UAC should prepare to receive SDP bodies in
F6 and/or F9, and just ignore them to handle a peer that does
not conform to the recommended implementation.
UAC UAS
| F1 INVITE (no SDP) |
|-------------------->|
| F2 1xx |
|<--------------------|
| |
| F3 1xx-rel (SDP) | <- The first 1xx-rel must contain SDP
|<--------------------| as the offer.
| F4 PRACK (SDP) | <- A PRACK request to the first 1xx-rel
|-------------------->| must contain SDP as the answer.
| F5 2xx PRA (no SDP) | -
|<--------------------| |
| | |
| F6 1xx-rel (no SDP) | <- The subsequent 1xx-rel should not
|<--------------------| | contain SDP.
| F7 PRACK | |
|-------------------->| | UAC can send a new offer in an UPDATE
| F8 2xx PRA | | request after F4.
|<--------------------| v
| |
| F9 2xx INV (no SDP) | <- The final response should not
|<--------------------| contain SDP.
| F10 ACK |
|-------------------->|
Figure 2 Example of Offer/Answer with 100rel Extension (2)
Note that in the case that the UAC needs to prompt the user to
accept or reject the offer, the reliable provisional response with
SDP as an offer (pattern 4) can result in the retransmission until
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the PRACK request can be sent. The UAC should take care to avoid
this situation when it sends the INVITE request without SDP.
3.2. Offer/Answer Exchange in Early Dialog
When both UAs support the 100rel extension, they can update the
session in the early dialog once the first offer/answer exchange
has been completed.
From a UA sending an INVITE request:
A UA can send an UPDATE request with a new offer if both ends
support the UPDATE method. Note that if the UAS needs to prompt the
user to accept or reject the offer, the delay can result in
retransmission of the UPDATE request.
A UA can send a PRACK request with a new offer only when
acknowledging the reliable provisional response carrying the answer
to an offer in the INVITE request. Compared to using the UPDATE
method, using PRACK can reduce the number of messages exchanged
between the UAs. However, to avoid problems or delays caused by
PRACK offer rejection, the UA is recommended to send a PRACK
request only when it has strong reasons to expect the receiver will
accept it. For example, the procedure used in precondition
extension [5] is a case where a PRACK request should be used for
updating the session status in an early dialog. Note also that if a
UAS needs to prompt the user to accept or reject the offer, the
delay can result in retransmission of the PRACK request.
From a UA receiving an INVITE request:
A UA can send an UPDATE request with a new offer if both ends
support the UPDATE method. A UAS can not send a new offer in the
reliable provisional response, so the UPDATE method is the only
method for a UAS to update an early session.
3.3. Offer/Answer Exchange in an Established Dialog
Both the re-INVITE and UPDATE methods can be used in an established
dialog to update the session.
The UPDATE method is simpler and can save at least one message
compared with the INVITE method. But both ends must support the
UPDATE method for it to be used.
The INVITE method needs at least three messages to complete but no
extensions are needed. Additionally, the INVITE method allows the
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peer to take time to decide whether it will accept a session update
or not by sending provisional responses. That is, re-INVITE allows
the UAS to interact with the user at the peer, while UPDATE needs
to be answered automatically by the UAS. It is noted that re-INVITE
should be answered immediately unless such a user interaction is
needed. Otherwise, some 3pcc flows will break.
4. Exceptional Case Handling
In RFC 3264 [3], the following restrictions are defined with regard
to sending a new offer.
"It MUST NOT generate a new offer if it has received an offer
which it has not yet answered or rejected. It MUST NOT generate
a new offer if it has generated a prior offer for which it has
not yet received an answer or a rejection."
Assuming that the above rules are guaranteed, there seem to be two
possible 'exceptional' cases to be considered in SIP offer/answer
usage: the 'message crossing' case, and the 'glare' case. One of
the reasons why the usage of SIP methods to exchange offer/answer
needs to be carefully restricted in the RFCs is to ensure that the
UA can detect and handle appropriately the 'exceptional' cases to
avoid incompatible behavior.
4.1. Message Crossing Case Handling
When message packets crossed in the transport network, an offer may
be received before the answer for the previous offer/answer
exchange, as described in Figure 3. In such a case, UA A must
detect the session description of the offer2 is not the answer to
offer1.
A B
|offer1 |
|----------------->|
| answer1|
|<------\ /-------|
| \/ |
| /\ offer2|
|<------/ \-------|
Figure 3 Message Crossing Case
When offer2 is in an UPDATE request or a re-INVITE request, a
session description cannot be the expected answer. Then UA A must
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reject the message including offer2 with a 491 response with Retry-
After header field.
When offer2 is in a PRACK request, that is, when the PRACK request
is acknowledging a reliable provisional response with an answer to
an offer in an INVITE request containing a session description, UA
A knows it is an offer. To avoid rejecting the PRACK or PRACK offer,
UA A is recommended to wait for answer1 before sending a PRACK
response with the answer to the offer2. Note that if UA A does not
send a new offer until the reliable provisional response with an
answer to the offer in the INVITE request is acknowledged with a
PRACK request, this case never happens. Fortunately, under the
current rules, UA A can not send a new offer until the reliable
response.
When offer2 is in a reliable provisional response or a successful
final response (as shown in Figure 4), UA A knows it is not the
answer to the offer1. For a reliable response to an initial INVITE
request, this case never happens. For a reliable response to a re-
INVITE request, UA A can infer that offer2 is not the answer1. In
this case, since UA A can not reject offer2 in a reliable response,
it is recommended that it wait for answer1 before sending a PRACK
request with the answer to offer2. Note that this case only occurs
when UA A, while waiting for an answer, sends an INVITE request
without session description.
Table 3 summarizes the discussions above.
offer2 | How to know it's not answer1 | Actions to take
-------+------------------------------+--------------------------
INVITE | Never be an answer | 491 response
UPDATE | Glare case for UA A | with Retry-After
-------+------------------------------+--------------------------
PRACK | This case never happens | -
| under the current rules. |
-------+------------------------------+--------------------------
1xx-rel| Only one INVITE transaction | Delay ACK/PRACK
2xx | at a time. Then UA can know | until answer1 is received
-------+------------------------------+--------------------------
NOTE: 1xx-rel/2xx case is extremely rare case and easily avoidable.
See Figure 4.
Table 3. UA's action to an offer (offer2) overtaking the previous
answer (answer1)
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A B
| |
|offer1(e.g. UPD) |
|==============================>|
|re-INV (no offer) |
|------------------------------>| --+
| answer1 (2xx-UPD)| |
|<===========\ /===============| | The first reliable response
| \/ offer2| |
| /\ (1xx-rel/2xx)| |
|<===========/ \===============| <-+
| answer2 (PRACK/ACK) |
|------------------------------>| Wait until answer1
| |
Figure 4 Reliable response as a message with offer2 in message
crossing case
4.2. Glare Case Handling
When both ends in a dialog send a new offer at nearly the same time,
as described in Figure 5, a UA may receive a new offer before it
receives the answer to the offer it sent. This case is usually
called a 'glare' case.
A B
|offer1 offer2|
|-------\ /-------|
| \/ |
| /\ |
|<------/ \------>|
Figure 5 Glare Case
When offer2 is in an UPDATE request or (re-)INVITE request, it must
be rejected with a 491 response.
When offer2 is in a PRACK request (within the current rules, only
possible if offer1 is in an UPDATE request), the PRACK may be
accepted with 200 or may be rejected with a 491 response. A 491
response is valid to satisfy the offer/answer model but it may
delay the completion of the reliable response transfer mechanism or,
in worst case, may result in the failure to complete the SIP
transaction because there is no clear retry rule when a PRACK
request is rejected with a 491 response. To avoid this glare
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condition, UA A should not send an offer if it has already sent a
reliable provisional response containing an answer to a previous
offer and has not received the corresponding PRACK request.
To avoid a glare condition involving an offer in a response, when
UA A has sent a (re)INVITE request without session description, it
should not send an offer until it has received an offer in a
reliable response to the (re)INVITE, and sent an answer to that
offer.
5. Content of Offers and Answers
While RFCs 3264[3] and 3312[5] give some guidance, questions remain
about exactly what should be included in an offer or answer. This
is especially a problem when the common "hold" feature has been
activated, and when there is the potential for a multimedia call.
Details of behavior depend on the capabilities and state of the
User Agent. The kinds of recommendations that can be made are
limited by the model of device capabilities and state that is
presumed to exist.
This section focuses on a few key aspects of offers and answers
that have been identified as troublesome, and will consider other
aspects to be out of scope. This section considers:
- choice of supported media types and formats to include and
exclude
- hold and resume of media
The following are out of scope for this document:
- NAT traversal and ICE
- specific codecs and their parameters
- the negotiation of secure media streams
- grouping of media streams
- preconditions
5.1. General Principle for Constructing Offers and Answers
A UA should send an offer that indicates what it, and its user, are
interested in using/doing at that time, without regard for what the
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other party in the call may have indicated previously. This is the
case even when the offer is sent in response to an INVITE or re-
INVITE that contains no offer. (However in the case of re-INVITE
the constraints of RFCs 3261 and 3264 must be observed.)
A UA should send an answer that includes as close an approximation
to what the UA and its user are interested in doing at that time,
while remaining consistent with the offer/answer rules of RFC3264[3] and other RFCs.
NOTE: "at that time" is important. The device may permit the
user to configure which supported media are to be used by
default.
In some cases a UA may not have direct knowledge of what it is
interested in doing at a particular time. If it is an intermediary
it may be able to delegate the decision. In the worst case it may
apply a default, such as assuming it wants to use all of its
capabilities.
5.2. Choice of Media Types and Formats to Include and Exclude5.2.1. Sending an Initial INVITE with Offer
When a UAC sends an initial INVITE with an offer, it has complete
freedom to choose which media type(s) and media format(s) (payload
types in the case of RTP) it should include in the offer.
The media types may be all or a subset of the media the UAC is
capable of supporting, with the particular subset being determined
by the design and configuration [6] of the UAC combined with input
from the user interface of the UAC.
The media formats may be all or a subset of the media formats the
UAC is capable of supporting for the corresponding media type, with
the particular subset being determined by the design and
configuration [6] of the UAC combined with input from the user
interface of the UAC.
Including all supported media formats will maximize the possibility
that the other party will have a supported format in common. But
including many can result in an unacceptably large SDP body.
5.2.2. Responding with an Offer when the Initial INVITE has no Offer
When a UAS has received an initial INVITE without an offer, it must
include an offer in the first reliable response to the INVITE. It
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has largely the same options as when sending an initial INVITE with
an offer, but there are some differences. The choice may be
governed by both static (default) selections of media types as well
as dynamic selections made by a user via interaction with the
device while it is alerting.
NOTE: The offer may be sent in a provisional response, before
the user of the device has been alerted and had an opportunity
to select media options for the call. In this case the UAS
cannot include any call-specific options from the user of the
device. If there is a possibility that the user of the device
will wish to change what is offered before answering the call,
then special care should be taken. If PRACK and UPDATE are
supported by caller and callee then an initial offer can be
sent reliably, and changed with an UPDATE if the user desires
a change. If PRACK and UPDATE are not supported then the
initial offer cannot be changed until the call is fully
established. In that case either the offer should be delayed
until the 200 is sent, or else the offer should include the
minimum set of media the user is able to select.
5.2.3. Answering an Initial INVITE with Offer
When a UAS receives an initial INVITE with an offer, what media
lines the answer may contain is constrained by RFC 3264.[3] The
answer must contain the same number of m-lines as the offer, and
they must contain the same media types. Each media line may be
accepted, by including a non-zero port number, or rejected by
including a zero port number in the answer. The media lines that
are accepted should typically be those that would have been offered
had the INVITE not contained an offer, excluding those not offered.
The media formats the answer may contain are constrained by RFC3264 [3]. For each accepted m-line in the answer, there must be at
least one media format in common with the corresponding m-line of
the offer. The UAS may also include other media formats it is able
to support at this time. However there is little benefit to
including added types.
If the UAS does not wish to indicate support for any of the media
types in a particular media line of the offer it must reject the
corresponding media line, by setting the port number to zero.
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Internet-Draft SIP Usage of the Offer/Answer Model March 20085.2.4. Answering when the Initial INVITE had no Offer
When a UAC has sent an initial INVITE without an offer, and then
receives a response with the first offer, it should answer in the
same way as a UAS receiving an initial INVITE with an offer.
5.2.5. Subsequent Offers and Answers
The guidelines above (sections 5.1. and 5.2.1. through 5.2.4. )
apply, but constraints in RFC 3264 [3] must also be followed. The
following are of particular note because they have proven
troublesome:
o The number of m-lines may not be reduced in a subsequent offer.
Previously rejected media streams must remain, or be reused to
offer the same or a different stream. (RFC 3264[3] section 6.)
o In the o-line, only the version number may change, and if it
changes it must increment by one from the one previously sent as
an offer or answer. (RFC 3264[3] section 8.) If it doesn't
change then the entire SDP body must be identical to what was
previously sent as an offer or answer. Changing the o-line,
except version number value, during the session is an error case.
The behavior when receiving such a non-compliant offer/answer
SDP body is implementation dependent. If a UA needs to negotiate
a 'new' SDP session, it should use the INVITE/Replaces method.
o In the case of RTP, the mapping from a particular dynamic
payload type number to a particular codec within that media
stream (m-line) must not change for the duration of the session.
(RFC 3264[3] section 8.3.2.)
NOTE: This may be impossible for a B2BUA to follow in some
cases (e.g. 3pcc transfer) if it does not terminate media.
When the new offer is sent in response to an offerless (re)INVITE,
all codecs supported by the UA are to be included, not just the
ones that were negotiated by previous offer/answer exchanges. The
same is true for media types - so if UA A initially offered audio
and video to UA B, and they end up with only audio, and UA B sends
an offerless (re)INVITE to UA A, A's resulting offer should re-
attempt video, by reusing the zeroed m-line used previously.
NOTE: The behavior above is recommended, but it is not always
achievable - for example in some interworking scenarios. Or,
the offerer may simply not have enough resources to offer
"everything" at that point. Even if the UAS is not able to
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offer any other SDP that the one currently being used, it
should not reject the re-INVITE. Instead, it should generate
an offer with the currently used SDP with o- line unchanged.
5.3. Hold and Resume of mediaRFC 3264 [3] specifies (non-normatively) that "hold" should be
indicated in an established session by sending a new offer
containing "a=sendonly" for each media stream to be held. An
answerer is then to respond with "a=recvonly" to acknowledge that
the hold request has been understood.
Note that the use of sendonly/recvonly is not limited to hold.
These may be used for other reasons, such as devices that are only
capable of sending or receiving. So receiving an offer with
"a=sendonly" must not be treated as a certain indication that the
offerer has placed the media stream on hold.
This model is based on an assumption that the UA initiating the
hold will want to play Music on Hold, which is not always the case.
A UA may, if desired, initiate hold by offering "a=inactive" if it
does not intend to transmit any media while in hold status.
The rules of RFC 3264 [3] constrain what may be in an answer when
the offer contains "sendonly", "recvonly", or "inactive" in an a=
line. But they do not constrain what must be in a subsequent offer.
The General Principle for Constructing Offers and Answers (section5.1. ) is important here. The initiation of "hold" is a local
action. It should reflect the desired state of the UA. It then
affects what the UA includes in offers and answers until the local
state is reset.
The receipt of an offer containing "a=sendonly" or "a=inactive" and
the sending of a compatible answer should not change the desired
state of the recipient. However, a UA that has been "placed on
hold" may itself desire to initiate its own hold status, based on
local input.
If UA2 has previously been "placed on hold" by UA1, via receipt of
"a=sendonly", then it may initiate its own hold by sending a new
offer containing "a=sendonly" to UA1. Upon receipt of that, UA1
will answer with "a=inactive" because that is the only valid answer
that reflects its desire not to receive media.
Once in this state, to resume a two way exchange of media each side
must reset its local hold status. If UA1 is first to go off hold it
will then send an offer with "a=sendrecv". The UA2 will respond
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with its desired state of "a=sendonly" because that is a permitted
response. When UA2 desires to also resume, it will send an offer
with "a=sendrecv". In this case, because UA1 has the same desire it
will respond with "a=sendrecv". In the same case, when UA2 receives
the offer with "a=sendrecv", if it has decided it wants to reset
its local hold but has not yet signaled the intent, it may send
"a=sendrecv" in the answer.
If UA2 has been "placed on hold" by UA1 via receipt of "a=inactive",
and subsequently wants to initiate its own hold, also using
"a=inactive", it need not send a new offer, since the only valid
response is "a=inactive" and that is already in effect. However,
its local desired state will now be either "inactive" or
"a=sendonly". This affects what it will send in future offers and
answers.
If a UA has occasion to send another offer in the session, without
any desire to change the hold status (e.g. in response to a re-
INVITE without an offer, or when sending a re-INVITE to refresh the
session timer) it should follow the General Principle for
Constructing Offers and Answers (section 5.1. ). If it previously
initiated a "hold" by sending "a=sendonly" or "a=inactive" then it
should offer that again. If it had not previously initiated "hold"
then it should offer "a=sendrecv", even if it had previously been
forced to answer something else. Without this behavior it is
possible to get "stuck on hold" in some cases, especially when a
third-party call controller is involved.
5.4. Behavior on receiving SDP with c=0.0.0.0RFC 3264[3] specifies that An agent MUST be capable of receiving
SDP with a connection address of 0.0.0.0, in which case it means
that neither RTP nor RTCP should be sent to the peer.
If a UA generates an answer to the offer received with c=0.0.0.0,
the direction attribute of the accepted media stream in the answer
must be based on direction attribute of the offered stream and
rules specified in RFC 3264 to form the a-line in the answer.
c=0.0.0.0 has no special meaning for the direction attribute of the
accepted stream in the answer.
6. Remaining Issues or Best Practices on Offer/Answer
This document clarifies the offer/answer usage in SIP and
summarizes the correct or recommended behaviors along with the
existing RFCs. To create any new normative behaviors beyond these
RFCs is not the intent of this document.
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However, through the scrutiny of the offer/answer model in SIP,
some issues are found to be unresolved within the current set of
RFCs. Those remaining issues are described in this section mainly
for further study.
6.1. Rejecting PRACK Offer
As stated in section 2.2. and 3.2. , it is recommended that an
offer not be sent in a PRACK request unless UAC has strong reasons
to assume the receiver will accept it. Even so, there may be cases
when the UAS has to reject the offer for some reason. The current
RFCs do not provide a way to reject the offer and at the same time
to indicate that the PRACK adequately acknowledged the reliable
response. It is unclear whether a non-200 response can still
indicate an acknowledgement of the reliable response.
Several ideas were presented to resolve this issue, such as sending
2xx PRACK response without SDP to reject the offer, or sending SDP
with a decreased version value in the o-line. Some of the
candidates may also be adapted as a way to reject an unacceptable
offer in a response. Anyway, those proposals violate the current
rules and lose backward compatibility to some extent (e.g. section5 of RFC 3262). It is beyond the scope of this document and remains
for further study.
The 488 response is another proposed solution; however the validity
and consequences of a 488 response to PRACK is an open issue.
Because the 488 response may be sent by a proxy, the UAC cannot
assume the reliable transaction has been adequately acknowledged.
If a 488 response is received, the UAC should ensure acknowledgment
of the reliable response by sending a new PRACK with the offer
removed or modified based upon the received 488 response. If the
488 response is sent by UAS (open issue), it cannot assume that the
UAC thinks that the reliable transaction has been adequately
acknowledged even though the UAS may treat otherwise (open issue).
If a 488 response is sent by UAS, the UAC should accommodate
receiving the altered PRACK with higher CSeq without expecting it
to trigger a 481 response (open issue).
NOTE: Deprecation of the usage of offer in PRACK may be
another solution. As the precondition mechanism specification
[2] explicitly shows a usage of sending offer in PRACK, its
deprecation could cause backward compatibility issues.
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Internet-Draft SIP Usage of the Offer/Answer Model March 20086.2. Commit/Rollback of Offer/Answer on Unsuccessful re-INVITE Transaction
When a re-INVITE transaction fails, the dialog remains with the
session bound to it. The issue here is: what is the session status
if an offer/answer exchange has been completed (if a session
description has been sent in a reliable provisional response to the
re-INVITE request), or if subsequent offer/answer exchanges have
taken place (using UPDATE or PRACK transactions), before the re-
INVITE transaction is terminated with a final error response
(Figure 6). One option is to take those offer/answer exchanges not
committed yet and to make the session status rollback to the one
before re-INVITE transaction was initiated. Another option is to
take those exchanges committed and to keep the session status as it
is even after re-INVITE fails. There is no clear consensus on which
one is the correct behavior.
There are some cases where it is useful to exchange
offer(s)/answer(s) even before re-INVITE completes. The case of
adding a new media (like adding video to audio only session) which
requires permission from the peer through some user interaction is
one example. Precondition procedures can be another case which may
require several offer/answer exchanges in one re-INVITE transaction.
UAC UAS
| session established |
|<===================>|
| |
| F1 re-INVITE (SDP) |
|-------------------->|
| F2 1xx-rel (SDP) |
|<--------------------|
| F3 PRACK | <- PRACK request may include new offer
|-------------------->| and can complete the offer/answer with
| F4 2xx PRA | the answer in 2xx PRACK response.
|<--------------------|
| | <- UPDATE method can update the session
| | status before receiving the final
| F5 4xx/5xx/6xx INV | response to re-INVITE request (F1).
|<--------------------|
| F6 ACK |
|-------------------->| Issue: What is the correct session
| | status after re-INVITE transaction.
Figure 6 Commit/Rollback Issue with re-INVITE transaction
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To make bad things worse, if a new offer from UAC and the final
response to re-INVITE are sent at nearly the same time, the UAS can
not know whether this new offer was sent before or after UAC
received the final failure response (Figure 7). Note that the ACK
request to the failure response is sent hop-by-hop basis, therefore
even after receiving the ACK request, UAS can not make sure that
UPDATE request was sent after the final response had been reached
to the other end.
Sending a new UPDATE request from UAC to synchronize the status
anytime after the re-INVITE fails may be a good option. This
solution, however, requires that the UPDATE method be supported by
both ends and needs care to avoid flapping when each end tries to
advertise their different views of the session status.
The proper handling of this issue is undefined by existing
standards. Resolution is beyond the scope of this document, and
will require a new normative document.
UAC UAS
| session established |
|<===================>|
| |
| F1 re-INVITE (SDP) |
|-------------------->|
| F2 1xx-rel (SDP) |
|<--------------------|
| F3 PRACK |
|-------------------->|
| F4 2xx PRA |
|<--------------------|
| |
|UPDATE(SDP) 4xx INV |
|---------\ /--------|
| \/ |
| /\ |
|<--------/ \------->|
| |
Figure 7 Commit/Rollback Issue with Race Condition
6.3. Offer in a Reliable Response
In RFC 3261, it is stated that when an INVITE is sent without an
offer, the first reliable response MUST contain an offer. There was
discussion on whether this rule can be loosened up. There is no
clear explanation why this restriction is defined. However, this
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rule will be left as it is, unless the strong necessity to loosen
it up is raised in the future.
6.4. Requesting Hold while already on HoldRFC 3264, section 8.4, contains procedures for putting a unicast
media stream on hold. Of particular note, it states:
"If the stream to be placed on hold was previously a recvonly
media stream, it is placed on hold by marking it inactive."
Section 5.3. of the current document makes a best practice
recommentation for this case which conflicts with that, and
explains why. Some concerns have been raised that such a
recommendation is invalid because RFC 3264 is normative on this
subject.
This document takes the position that Section 8.4 of RFC 3264 is
non-normative, and so may be overridden. It is further recommended
that RFC 3264 be revised to avoid the confusion.
7. Add New Offer/Answer Usage in SIP
This document recommends against the addition of new offer/answer
methods using SIP. However, it may be necessary to define new
offer/answer exchange methods as SIP extensions evolve. This
section recommends some things that should be taken into
considerations in that case.
7.1. Explicit Usage
New method definitions should define offer/answer usage explicitly
without any ambiguity.
7.2. Rejection of an Offer
New method definitions should define how to reject an offer where
possible.
7.3. Backward Compatibility
New methods must keep backward compatibility.
7.4. Exceptional Case Handling
New methods should take care of how to handle exceptional cases,
message crossing case and glare case.
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Author's Addresses
Takuya Sawada
KDDI Corporation
3-10-10, Iidabashi, Chiyoda-ku, Tokyo, Japan
Email: tu-sawada@kddi.com
Paul H. Kyzivat
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719
USA
Email: pkyzivat@cisco.com
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Internet-Draft SIP Usage of the Offer/Answer Model March 2008
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